A depth generation method adapted for a 2D to 3D image conversion device is provided. The depth generation method includes the following steps. Motion vectors in an image frame are obtained by motion estimation. A global motion vector of the image frame is obtained. Motion differences between the motion vectors of each block and the global motion vector are calculated. A depth-from-motions of each block is obtained based on the motion differences. Furthermore, a depth generation apparatus using the same is also provided.